The invention relates to a device capable of clamping, and preferably also sealing, the polish rod of a pumping wellhead.
A conventional wellhead comprises an assembly of spools, valves and assorted adapters which provide pressure control of a production well. In a rotary pumping oil well typical components include, from the bottom up:
a flanged casing head attached to the well casing;
a flanged tubing head having an internal hanger from which the well tubing string is suspended;
a tubing head adapter having a flanged connection at its bottom end and a threaded connection of smaller diameter at its top end;
a production blowout preventer (BOP) body having top and bottom threaded connections and including side openings for receiving the BOP ram components;
a flow tee body having threaded bottom and top connections and a threaded or flanged side opening for connecting with a flow line;
a polish rod stuffing box; and
a rotary drive assembly for rotating the well's rod string to power a downhole progressive cavity pump.
These components, except for the rotary drive assembly, combine to form a vertical central bore extending through the pressure-containing wellhead body members. The polish rod of the rod string extends through this central bore in the pressure zone maintained in the wellhead body members. The polish rod rotationally drives a drive string or “sucker rods” which in turn drive the progressing cavity pump located at the bottom of the installation to produce well fluids to the surface through the wellhead.
The combination of the tubing head adapter, BOP body and flow tee body components is commonly collectively referred to as a “pumping tree.” The assembly of wellhead components above the tubing head is usually referred to collectively as the “Christmas tree.”
Composite wellhead assemblies exist which include, between a top connector and bottom connector, one or more of the following components, in any sequence, adapter, valve, gate valve, flow tee, blowout preventer, and polish rod clamp. To that end, attention is directed to the devices disclosed in the following U.S. Patents, all of which are owned by Stream-Flo Industries Ltd., the assignee of the present application:
U.S. Pat. No. 5,743,332, issued Apr. 28, 1998, entitled “Integral Wellhead Assembly for Pumping Wells. This patent teaches integrating the tubing head adapter, BOP body and flow tee body into a unitary structure, referred to as an ‘integral or composite pumping tree’, by forging, casting or machining a single steel body. This composite pumping tree forms the lower end of the Christmas tree.
U.S. Pat. No. 6,457,530, issued Oct. 1, 2002, entitled “Wellhead Production Pumping Tree”. This patent is similar to U.S. Pat. No. 5,743,332, but includes a Y-arm to provide access for coiled tubing.
U.S. Pat. No. 6,176,466, issued Jan. 23, 2001, entitled “Composite Pumping Tree with Integral Shut-Off Valve. This patent application teaches integrating a tubing head adapter, shut-off valve body, BOP body, and flow tee body into a composite pumping tree.
U.S. Pat. No. 6,595,278, issued Jul. 22, 2003, entitled “Assembly for Locking a Polished Rod in a Pumping Wellhead”. This patent includes multiple embodiments to incorporate a polish rod locking device in a composite pumping tree.
Each of the above patents discloses wellhead equipment used in connection with pumping oil wells, but in a composite form, meaning that one or more functional components of a conventional pumping tree are included in an integral pressure-containing wellhead body between a top and a bottom connector.
Canadian Patent No. 2,349,988 to Oil Lift Technology Inc. issued on May 7, 2002, discloses a polish rod lock out clamp for securing the polish rod in an oil well installation. In one embodiment, the polish rod lock out clamp is integrated with a blowout preventer. The pistons, or BOP rams, are of metal and include an arcuate recess (or radial groove) at their front or inner ends to accommodate and grip the polish rod in metal to metal contact. An elastomeric seal across the arcuate recess is provided to seal against the polish rod. To grip the polish rod, the arcuate recess has a diameter which is undersize relative to that of the polish rod. However, the smaller diameter of the arcuate recess in the ram can cause difficulty in aligning and gripping of an off-centre polish rod. As well, with repeated use in gripping the polish rod, the polish rod becomes scored or damaged as the clamp bites into the polish rod to achieve a sufficient gripping action. Damaging the polish rod in this manner can effectively reduce the diameter of the polish rod in this area, making it more difficult to clamp. Eventually, the damage to the polish rod can lead to an inability to seal against the downhole pressure in this area.
U.S. Pat. No. 7,000,888, issued Feb. 21, 2006, naming Andrew Wright et. al., as inventors, discloses a polish rod clamp or combination BOP/clamp for gripping a polish rod, without requiring the metal to metal contact set forth in the above Oil Lift patent. The clamping members in the Wright et al. patent application include gripping inserts, which may be coated with non-metallic ceramic materials, to clamp the polish rod without scoring or damaging the rod surface. This patent application still includes the arcuate recess (generally semi-circular in cross section) at the front ends of the clamping members, which can cause difficulties as set forth above in aligning with an off-centre polish rod.